Which TV jack in your house should be used by your cable modem?

Yes. "You want to use the jack that connects directly to the point where the TV cable comes into you home with no splitters between this jack and the entrance point. Later, if your cable modem connection is constantly going down, you might consider that you've chosen the wrong jack for the connection because an inline splitter can degrade the connection. The cable company that installed the jacks should be able to tell you which jack is best to use for the cable modem." Source: CompTIA A+ Guide to Managing & Maintaining Your PC, Sixth Edition, Jean Andrews, P. 934